The invention relates in general to munitions and, in particular, to large caliber gun tubes, such as cannons.
Bore evacuators may be used on large caliber cannons. Bore evacuators may be used to remove propellant gases from the gun bore of the cannon after firing. The gun may be attached to a vehicle that may be manned. The rear of the gun may include a breech. The breech may be located inside a crew compartment. Flash-back in the crew compartment may be a hazard. Exposure of the crew to toxic propellant gases may be a hazard.
Some bore evacuator systems may use a reservoir to store high pressure propellant gases. Ports or nozzles may be located in the gun tube to charge and discharge the reservoir. The gun may include a means to quickly open the breech of the gun. The breech may be opened shortly after firing the gun, on return of the gun to battery.
A bore evacuator may use an ejector principle. The reservoir may be charged with high pressure gas as soon as the projectile passes the ports in the gun tube. When pressure in the gun tube falls below the pressure in the reservoir, the high pressure gas may begin to exit the reservoir through the ports in the gun tube. Gas may begin exiting the reservoir about 20 to 50 milliseconds after firing of the gun. As the gas flow begins to exit the ports, the breech of the gun may not be open.
At approximately 500 milliseconds after firing the gun, the breech may be opened. The breech may be opened by an automatic action as the gun returns to battery. As a result of the breech not opening for about 400-500 milliseconds after gas begins to flow out of the reservoir, no fresh air may be drawn into the gun tube. As a result, no propellant gases may be expended from the gun tube. The ports between the gun tube and the reservoir may not provide an ejector action to the propellant gases. The lack of ejector action may be because the pressure difference or suction provided by the ports may not be equalized by drawing fresh air in the gun tube.
When the breech does open, most of the gas may have been exhausted from the reservoir. What little gas pressure remains in the reservoir may then be used to provide ejector action to the gases in the gun tube. As the gas stored in the reservoir flows from the reservoir through the ports and into the gun tube, the stored gas may entrain the propellant gas inside the gun tube and push the propellant gas out of the muzzle end of the tube. As a result, fresh air may be drawn into the gun tube through the open breech. A known ejector system used in a bore evacuator may draw about 10 to 15 times as much gas through the gun tube as the amount of stored gas that may be injected from the reservoir through the ports in the side of the gun tube. Because of the ejector mechanism, bore evacuators may be an efficient mechanism to evacuate propellant gases from a gun bore.
Because of the long delay of about 400-500 milliseconds between when the stored gas begins to flow from the reservoir and when the breech opens, various methods for improving the performance of bore evacuators have been tried. One method may be to increase the reservoir size such that a sufficient amount of stored gas remains in the reservoir when the breech is opened. Using this method, bore evacuators may be significantly oversized.
Another method may be to provide a larger intake port cross-sectional area into the reservoir compared to the discharge port cross-sectional area out of the reservoir. Intake port cross-sectional area may be increased by adding more charging ports that contain check valves.
Another approach may be to use a small high pressure chamber that has a check valve connected to a low pressure chamber. The intake charging ports may be connected to the high pressure chamber and the discharge ports may be connected to the low pressure chamber. As a result, a higher charge volume and/or mass may be obtained for a given discharge port charge area. The higher charge volume or mass may provide a higher pressure at the discharge ports during discharge of the evacuator.
An important factor may be the ratio of the total cross-sectional area of the charge ports to the total cross-sectional area of the discharge ports. As that ratio increases, the amount of discharge time may also increase. Simply increasing the volume of the stored gas may not be effective if the total cross-sectional discharge area is large.
Large caliber guns may operate with vastly differing propellant charges. A single artillery weapon may use different charge configurations. As a result, the bore evacuator may need to function properly over a wide range of charge pressures. Typically, the bore evacuator may be significantly over-designed to properly function at both low and high charge pressures. Thus, bore evacuators may be rather large and heavy devices.
A need exists for a bore evacuator that may be of a small volume, while providing effective bore evacuation of the gun.